Power-synchronisation control (PSC), also known as synchronous-machine emulating control or virtual synchronous machine, was originally invented for the control of a voltage-source converter (VSC) in high-voltage, direct current (HVDC) light transmission where both terminals were situated in very weak ac networks. PSC is disclosed in EP2319170. PSC can be used for any grid-connected VSC, not just HVDC.
The name PSC refers to how the method achieves synchronization to the grid; via the control of the active power. PSC therefore does not utilize a phase-locked loop (PLL). The core of PSC is the power-control law, which sets the phase angle of the converter voltage. With the power direction into the converter, i.e. from the AC grid, the phase angle can be expressed as:θ=∫[ω1+Kp(P−Pref)]dt  (1)where ω1 is the nominal angular grid frequency, Kp is the power control gain, P is the active power input to the converter and Pref is the active power reference.
An inner current controller can be added to the power-control law, as disclosed in EP2319170. In this manner, the converter current can be limited in order to prevent overcurrent, particularly during faults. A drawback with this method is that synchronism may be lost when the inner current controller becomes active.